Nickel-based and cobalt-based superalloy materials are commonly used to provide high mechanical strength for very high temperature applications, such as for the blades or other components of a gas turbine engine. The term “superalloy” is used herein as it is generally understood in the art to refer to alloys that can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature of the material. Creep and oxidation resistance are primary design criteria for such materials. Examples of superalloys are those sold under the trademarks/commercial designations Hastelloy, Inconel, IN939, Haynes-188, MP98T, TMS-63, TMS-71, TMS-75, and CM-247 LC, and those described in U.S. Pat. Nos. 7,011,721; 7,004,992; 6,974,508; 6,936,116; and 6,494,971; among many others.
Superalloy components are very expensive, and thus the repair of a damaged part is preferred over its replacement. However, known weld repair techniques for superalloy materials have met with only limited success, due primarily to the propensity of superalloy materials to develop cracks resulting from low melting temperature grain boundary constituents and the volume changes that occur during cooling due to the precipitation of a gamma prime phase. Some high strength superalloys containing large percentages of gamma prime phase, such as the material sold under the commercial designations Mar-M 247 and CM247LC, are considered almost impossible to weld due to their propensity to form microcracks in the region under the weld bead and in the heat affected zone (HAZ). In addition to such hot cracking of the weld filler metal and heat affected zone, these materials exhibit strain age cracking, which results in cracks extending into the base metal of the component. Strengthening elements used in superalloys include: elements which segregate primarily to the gamma phase for solid solution strengthening (W, Mo, Re, Cr, Co, Fe); elements which segregate primarily to the gamma prime phase (Al, Ti, Nb, Ta, V); and grain boundary strengtheners (C, B, Zr, Hf, Y). FIG. 6 illustrates the known relationship between weldability and the weight percentage of the specific strengthening elements aluminum and titanium. Superalloys are generally readily weldable when the sum of the concentration of aluminum plus one half of the concentration of titanium is less than 3% by weight (w/o). This region of FIG. 6 may be referred to herein as the zone of weldability. If that sum is greater than 3 w/o, the superalloy is generally difficult to weld.
Several techniques have been proposed to improve the weldability of superalloy materials. U.S. Pat. No. 4,336,312 describes a combination of a controlled chemical modification of a cast nickel-based superalloy material along with a pre-weld thermal conditioning cycle. Other known techniques utilize a very high density power source (laser or e-beam) to form a very small weldment in order to limit the amount of melted material. One such technique is disclosed in U.S. Pat. No. 6,364,971 which describes a laser welding technique following a pre-conditioning hot isostatic process. U.S. Pat. No. 6,333,484 describes a welding technique wherein the entire weld area is preheated to a maximum ductility temperature range, and this elevated temperature is maintained during the welding and solidification of the weld. U.S. Pat. No. 7,051,435 describes the use of a braze preform for forming a lower temperature brazed joint with a superalloy material. Each of these patents is incorporated by reference herein. Further improvements in the welding of superalloy materials and other difficult-to-weld alloys are desired.